What is the diameter difference between 275/40R18 and 295/30R20?

275/40R18 has an overall diameter of 26.66 inches (677 mm). 295/30R20 is 26.97 inches (685 mm). That's a 0.31 inch (8 mm) difference — 295/30R20 is taller.

Will 295/30R20 cause speedometer error on a car fitted with 275/40R18?

Swapping 275/40R18 for 295/30R20 produces a speedometer error of about 1.2%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 60.7 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

275/40R18 turns 756 revs per mile. 295/30R20 turns 748 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

275/40R18 has a sidewall of 110 mm (4.33 in). 295/30R20 sidewall is 89 mm (3.48 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 295/30R20 a safe plus-size upgrade from 275/40R18?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 1.2%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

275/40R18 vs 295/30R20

Overall diameter, sidewall, revs per mile, and speedometer error — 295/30R20 is the taller tire, with a 1.2% speedo delta when swapping.

275/40R18

26.66in

677 mm overall diameter

295/30R20

26.97in

685 mm overall diameter

Diameter Δ

0.31 in

Speedo error

1.2%

275/40R18 revs/mi

756

295/30R20 revs/mi

748

Within the ±3% overall-diameter band usually considered safe for plus-sizing — speedometer and ABS calibration should stay close to factory.

Full spec

Side by side

Metric	275/40R18	295/30R20
Section width	275 mm	295 mm
Aspect ratio	40	30
Wheel diameter	18″	20″
Sidewall height	110 mm	89 mm
Overall diameter	26.66 in (677 mm)	26.97 in (685 mm)
Circumference	83.76 in	84.72 in
Revolutions / mile	756	748

Other comparisons for these sizes

255/40R18 vs 275/40R18 275/35R18 vs 275/40R18 275/40R18 vs 275/35R19 275/30R20 vs 295/30R20 285/30R20 vs 295/30R20 285/35R20 vs 295/30R20 295/30R20 vs 305/30R20 295/30R20 vs 295/35R20

FAQ

Frequently asked

What is the diameter difference between 275/40R18 and 295/30R20?: 275/40R18 has an overall diameter of 26.66 inches (677 mm). 295/30R20 is 26.97 inches (685 mm). That's a 0.31 inch (8 mm) difference — 295/30R20 is taller.
Will 295/30R20 cause speedometer error on a car fitted with 275/40R18?: Swapping 275/40R18 for 295/30R20 produces a speedometer error of about 1.2%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 60.7 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 275/40R18 turns 756 revs per mile. 295/30R20 turns 748 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 275/40R18 has a sidewall of 110 mm (4.33 in). 295/30R20 sidewall is 89 mm (3.48 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 295/30R20 a safe plus-size upgrade from 275/40R18?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 1.2%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.